Vinyl aromatic resins, such as polystyrene, for instance, have been found to be useful for thermoplastic molding compositions. However, these resins have poor heat distortion and impact resistance properties, and attempts have been made to upgrade these properties. One approach has been to modify the vinyl aromatic resins by copolymerizing these materials with .alpha.,.beta.-unsaturated cyclic anhydrides to form copolymers such as poly(styrene-maleic anhydride). Although improvements in heat resistance and solvent resistance are provided, the resulting copolymers are somewhat brittle and do not have good resistance to impact.
To improve the impact resistance of copolymers of vinyl aromatic resins and .alpha.,.beta.-unsaturated cyclic anhydrides, the copolymers have been blended with elastomers. For instance, blends of nitrile rubber and styrene-maleic anhydride copolymers are disclosed in U.S. Pat. Nos. 2,914,505 and 3,641,212. With some of these compositions, however, the components are not compatible and the compositions are difficult to prepare.
In copending U.S. application Ser. No. 477,435, filed June 7, 1974, and assigned to the same assignee as herein, thermoplastic molding compositions are disclosed which comprise an unmodified copolymer of a vinyl aromatic compound and an .alpha.,.beta.-unsaturated cyclic anhydride, and an elastomeric linear block copolymer of a vinyl aromatic compound and a diene. The components are compatible over a wide range of proportions and the resulting compositions provide molded articles having good mechanical properties.
It has now been surprisingly discovered that rubber-modified copolymers of a vinyl aromatic compound and an .alpha.,.beta.-unsaturated cyclic anhydride can be compatibly admixed with a copolymer of a vinyl aromatic compound and a conjugated diene, to form compositions which are moldable to finished articles of unexpectedly improved mechanical properties. The compositions of this invention provide articles having improved impact strength, tensile properties, and the like, when compared with the compositions of the above-mentioned U.S. Ser. No. 477,435.